The largest study yet to examine the risks of hormone-based birth control has concluded the contraceptives carry a small risk of stroke and heart attack, depending on the method and type of hormone used.
But the risk for individual women remains extremely low, particularly in younger women, a team of Danish researchers reported.
Their findings, published this week in the New England Journal of Medicine, suggest a higher risk of stroke in particular for women using vaginal rings, and possibly hormonal skin patches – though the second finding was based on a smaller group of women and could have been due to chance.
Other factors – such as the belief that a patch or a ring might be safer for women thought to be at risk of stroke – may explain the higher rate of stroke in that group, said James Simon, a women's health researcher at George Washington University in Washington, who was not involved in the study.
Dr. Simon said the findings probably should not change how doctors prescribe birth control, noting that the risks seen in the study pale in comparison with the risks of stroke, heart attack or death faced by women who get pregnant.
"None of the hormonal contraceptives studied were associated with an excess risk of stroke that was unacceptable, considering their contraceptive and non-contraceptive benefits," Diana Petitti of Arizona State University in Tucson wrote in an editorial accompanying the study.
Previous attempts to assess the risk of stroke or heart attack linked with hormonal contraceptives have produced conflicting results.
In the latest study, a team led by Ojvind Lidegaard from Copenhagen University Hospital analyzed the records of 1.7 million Danish women aged 15 to 49 to assess the potential dangers associated with their contraceptive use. None of the women had a history of heart disease or cancer.
They were followed for 15 years beginning in 1995.
Overall, the risk to individual women was small. One in every 4,700 women had a stroke each year and one in every 9,900 suffered a heart attack.
Women taking contraceptive pills with a combination of estrogen and progestin tended to have a higher risk of stroke and heart attack than those not using hormonal contraception.
For some hormone combinations, that difference could have been due to chance, but women using estrogen with norethindrone or desogesterel at certain doses, for example, had double the risk of both complications compared with non-users.
Still, Dr. Simon said the new research "shows very little difference between the different pills for the same dose of estrogen, which will make women's choices larger."
Dr. Lidegaard said in an interview that many women have used patches and rings "believing that these non-oral products could confer less risk. But this is definitely not the case."
He said age is a key factor when considering risks.
"If you are 20 years old and you double your risk of [stroke], then you still have a very low risk because the absolute risk is so low," he said.
"On the other hand, if you are in the other path of reproductive age, especially in the 40s, you should consider not increasing your risk ... further because it's already increased due to your age."
Among all women, once they stopped using hormonal contraception, their risk of strokes and heart attacks dropped to the same level as the risk for women who had never used them.